The expansion of the green roof industry in North America has increased annually and grew by 115% in 2011 alone [Green Roofs for Healthy Cities (GRHC), 2012]. With the increasing area covered by installed green roofs, production demand has also increased for green roof plants. Given the environmental, economic, and aesthetic benefits provided by green roofs (Barker and Lubell, 2012; Berndtsson, 2010; Getter and Rowe, 2006; Oberndorfer et al., 2007) as well as the billions of square feet for potential green roof installations in North America (GRHC, 2012), there is great potential for future growth of plant production for the North American green roof industry. In 2011, the majority of green roof installations were extensive green roofs (GRHC, 2012), which typically consist solely or mainly of Sedum spp. planted in a growing substrate depth of 15 cm or less (FLL, 2008). Sedum are ideal plants for extensive green roof systems as a result of their ability to grow in shallow substrates (Emilsson, 2008; Rowe et al., 2012) and endure environmental stresses (Durhman et al., 2006; Getter and Rowe, 2006; Wolf and Lundholm, 2008). Thus, efficient production of sedum as plants, cuttings, and pre-vegetated extensive green roof systems (e.g., modules) is required to meet the needs of the green roof industry.
Similar to ornamental crop production (i.e., Salifu and Jacobs, 2006; Zhang et al., 2012), adequate fertility is essential for green roof plant production to encourage plant growth, ensure desired aesthetics, and achieve full vegetative coverage (Clark and Zheng, 2012, 2013; Emilsson et al., 2007; FLL, 2008; Retzlaff et al., 2009). Green roof growing substrates used during production are markedly different (i.e., lightweight, well-drained and 40 g·L−1 or less organic matter; FLL, 2008) than those used in other horticultural sectors and may require unique fertilizer types and rates. Commercially, sedum-vegetated modules propagated in May with 976 g of cuttings per m2 are produced in roughly 10 to 12 weeks based on 95% vegetative coverage at production completion (Barker and Lubell, 2012). It is desirable for growers to minimize module production times, as evaluated primarily by overall vegetative coverage, to increase the quantity of modules produced and, consequently, farm-gate income. In addition, sedum can be grown to produce cuttings for module propagation or direct-to-roof plantings. We have not found literature studying sedum cutting production or the combination of sedum cutting harvest with finished green roof module production, although pruning, clipping, or pinching increases the density and vigor of groundcover plants (Klingeman et al., 2008).
Vegetative coverage and shoot growth for sedum in green roof systems have demonstrated positive responses to CRF rate both during production of commonly installed modules and post-installation module maintenance (Barker and Lubell, 2012; Clark and Zheng, 2013; Emilsson et al., 2007). Building on previous green roof module production results (Barker and Lubell, 2012), research is needed to determine CRF rate recommendations for sedum-vegetated module production, considering additional growing substrates, fertilizer types and rates, and Sedum species combinations. In addition, nutrient runoff (i.e., leaching) to the environment needs to be evaluated and compared with regional water discharge recommendations (e.g., Canadian Council of Ministers of the Environment, 2012; Ministry of the Environment and Energy, 1994) before conscientious CRF rate recommendations can be made. Fertility of the growing substrate influences nutrient leaching (Alsup et al., 2011; Gregoire and Clausen, 2011; Vijayaraghavan et al., 2012) and new green roof plantings leach more nutrients than established plantings (Emilsson et al., 2007). Therefore, CRF rate in combination with the growing substrate fertility should be considered when determining optimum CRF rates for newly propagated green roof systems. Although past fertilization studies have evaluated nutrient runoff during vegetation establishment in a greenhouse (Emilsson et al., 2007) and plant growth and coverage during outdoor production (Barker and Lubell, 2012), to our knowledge, no research has considered both the optimum CRF rate for green roof module production in combination with nutrient loss through leaching in an outdoor trial. Determining an optimum CRF rate for sedum growth in green roof modules while minimizing nutrient leaching will help growers develop environmentally conscious and efficient production systems to meet industry demand.
The current study aims to evaluate the effect of CRF applied during outdoor production of green roof modules. Specifically, the objective of this study was to identify an optimum CRF rate to minimize both green roof module production time and nutrient leaching to the environment.
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